Temporal variation of nitrogen balance within constructed wetlands treating slightly polluted water using a stable nitrogen isotope experiment
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Slightly polluted water has become one of the main sources of nitrogen contaminants in recent years, for which constructed wetlands (CW) is a typical and efficient treatment. However, the knowledge about contribution of individual nitrogen removal pathways and nitrogen balance in constructed wetlands is still limited. In this study, a stable-isotope-addition experiment was performed in laboratory-scale constructed wetlands treating slightly polluted water to determine quantitative contribution of different pathways and temporal variation of nitrogen balance using Na15NO3 as tracer. Microbial conversion and substrate retention were found to be the dominant pathways in nitrogen removal contributing 24.4–79.9 and 8.9–70.7 %, respectively, while plant contributed only 4.6–11.1 % through direct assimilation but promoted the efficiency of other pathways. In addition, microbial conversion became the major way to remove N whereas nitrogen retained in substrate at first was gradually released to be utilized by microbes and plants over time. The findings indicated that N2 emission representing microbial conversion was not only the major but also permanent nitrogen removal process, thus keeping a high efficiency of microbial conversion is important for stable and efficient nitrogen removal in constructed wetlands.
KeywordsConstructed wetlands Stable isotope Nitrogen balance Temporal variation N2 and N2O emission Elodea nuttallii and Iris sibirica
We would like to sincerely thank Prof. Jinbo Zhang and his team for providing valuable advices and help on 15N abundance analysis. This study was financially supported by research projects of the Major State Water Pollution Control and Treatment Technique Programs of China (No. 2012ZX07101006 and No. 2013ZX07101014-001).
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Conflict of interest
The authors declare that they have no competing interests.
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